Curved static eliminator

ABSTRACT

A device for the removal of electrostatic charge from a charged body, the device comprising (a) a frame, and (b) a radioactive source positioned within the frame; the frame and the radioactive source having a curvature in the long axis thereof and forming a structure adapted to coaxially encircle at least part of the charged body, and said device being adapted to emit radiation in a direction toward the charged body.

United States Patent [191' Lindsay et al.

1 1 CURVED STATIC ELIMINATOR [75] Inventors: Thomas W. Lindsay, St. Anthony;

Donald M. Yenni, J r., St. Paul, both of Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn.

[22] Filed: Mar. 20, 1972 [21] Appl; No.: 236,117

Related US. Application Data [63] Continuation-impart of Ser. No. 52,947, July 7,

1970, abandoned.

[52] US. Cl 317/2 R [51] Int. Cl. 1105i 3/00 [58] Field of Search 317/2 R Feb. 19, 1974 [56] References Cited UNITED STATES PATENTS 2,048,490 7/1936 Bilstein 317/2 R 2,479,882 8/1949 Wallhausen et al.... 317/2 R 2,752,533

6/1956 Maas 317/2 R Primary Examiner-L. T. Hix

[5 7] ABSTRACT A device for the removal of electrostatic charge from a charged body, the device comprising (a) a frame, and (b) a radioactive source positioned within the frame; the frame and the radioactive source having a curvature inthe long axis thereof and forming a structure adapted to coaxially encircle at least part of the charged body, and said device being adapted to emit radiation in a direction toward the charged body.

7 Claims, 4 Drawing Figures v CURVED STATIC .ELIMINATOR This application is a continuation-in-part of our copending application Ser. No. 52,947, filed July 7, 1970, and now abandoned.

This invention relates to a curved, radioactive static eliminator device for removing an electrostatic charge from the surface of a curved or irregular shaped charged body.

It is well known that in various processing operations where sheet material or other material is moved in frictional engagement with guide rolls, guide bars,- and processing equipment, undesirable static electric charges develop. The frictional engagement between the moving body and a different-material causes the creation of static electrical charges on the surface of the body which may accumulate to high levels. Where the body is a flat sheet, for example, paper, plastic film, etc., ordinary linear or straight static eliminator devices may be used to eliminate the static charge from the sheet. However, where the charged body has a curved or irregularly shaped surface, a flat or straight static eliminator device cannot remove the static charge as effectively.

Another processing operation in which linear static eliminator devices are not very effective is that of filling a plastic tube or plastic bag with fine powdered material. In such operation any electrostatic charge on the tube or bagwill interfere with the flow of powdered material into the bag or through the tube. As a result, the control of flow of the powdered material may be very inefficient. The charge may also cause powder to remain adhered to the interior walls of the bag, thus rendering heat sealing operations ineffective.

Although there are several types of static eliminators known to those familiar with the art, e.g., electric powered devices, nuclear powdered devices, needle induction bars, etc., such static eliminator devices are not adapted to effectively remove the electrostatic charge from curved or irregularlyshaped charged bodies. Some types ofinduction static eliminators (e.g., copper bristle rope) may be formed around the surface of a charged body, but such induction devices have inherent limitations depending upon the initial voltage on the charged body. That is, an induction static eliminator is effective for reducing the static charge from several thousand volts to a level of about 5,000 volts, but if the initial voltage on the charged body is 5,000 volts or less, an induction static eliminator is effectively inoperative. In various processing operations, it is necessarythat the static charge be reduced to essentially zero volts. However, the induction static eliminators do not achieve such results unless the initial voltage is very high (e.g., 50,000 volts). Electrically powered static eliminators are effective for reducing the static charge to essentially zero volts, but in order for those static eliminator bars to completely reduce the charge from a curved or irregular shaped body a plurality of elimi- V nators would have to be used.

Heretofore others have eliminated static electricity from curved or irregularly shaped bodies by positioning a number of noncurved elongated static eliminators (e.g., electrically powered static eliminators) around the charged body while others have used blowers to blow large volumes of ionized air or gas around the charged body to remove the static charge. These approaches, however, have been found unsatisfactory for a number of reasons. For example, the use of ionized air blowers produces air currents and attendant dust problems. Also, it is often very difficult to position a plurality of elongated static eliminators in close relation to a charged body. Furthermore, the use of noncurved static eliminators and streams of ionized air typically results in non-uniform static removal from an irregularly shaped charged body.

The present invention provides an effective and relatively inexpensive static eliminating device for the removal of electrostatic charges from the surface of a curved or irregularly shaped charged body.

In accordance with the invention there is provided a device for the removal of electrostatic charge from a charged body, said device comprising, (a) a frame, and (b) a radioactive source positioned within said frame; said frame and said radioactive source having a curvature in the long axis thereof and forming a structure adapted to coaxially encircle at least part of said charged body, said structure having a curvature which is at least semi-circular, and said device being adapted to emit radiation in a direction toward said charged body. The invention is described in more detail hereinafter with reference to the accompanying drawings wherein like reference characters refer to the same part throughout the several views, in which:

FIG. 1 is a schematic view of a curved static eliminator of the invention surrounding a charged body;

FIG. 2 is a perspective view of one embodiment of a curved static eliminator;

FIG. 3 is a cross-sectional view along line 3-3 of FIG. 2; and

FIG. 4 shows another embodiment of a curved static eliminator of the invention.

Referring first to FIG. 1 to illustrate the effectiveness of a curved static eliminator device, there is shown a schematic view of a curved static eliminator device 10 which completely surrounds and coaxially encircles a charged body 50. The static eliminator device 10 has a radius R. There is shown an arc distance S which subtends an angle 6. As shown in FIG. 1, charged body 50 has a radius of RA. Arc length L of body 50 also subtends an angle equal to 0. Thus, the radioactivity emit-' ted by the length S of the static eliminator l0 converges upon and ionizes the air along length L of the charged body 50. When 0 is expressed in radians the relationship between S, R, and 0 can be expressed by the formula S=R6, and the relationship between L, R, A, and 6 can be expressed by the formula L=(R-A)6.

The effectiveness of the curved static eliminator can now be readily compared with the effectiveness of a straight or flat static eliminator. A straight or flat bar of S units of length would cause an amount of air ionizationX over a linear distance equal to the length of the straight bar, i.e., over a length of S units. Thus, the air ionization would be distributed over a charged body at a concentration of X ionization units/S length units. A curved static bar ofS units oflength, however, would distribute X ionization units over a length of L'units, as shown in FIG. I. To determine relative effectiveness of the curved and the straight eliminator, the concentration of air ionization on the surface at a charged body provided by the curved bar is divided by the concentra tion of air ionization on the surface of a charged body provided by a straight bar, i.e.,

Thus, the curved bar can provide l/l l(A/R)] times more air ionization units to the surface of a charged body than the straight bar provides.

Thus, because the curved static eliminator converges the radiation emitted therefrom onto the surface of the charged body, the curved static eliminator is therefore much more effective for reducing static charges than is a straight or flat static bar. As can be seen from FIG. 1, the curved static bar is'most effective when it is annular in shape and coaxially positioned with respect to the charged body. However, it is not essential that the static bar be of annular shape, For example, an elliptical or polygonal shaped bar would also be effective for reducing the static charge on irregularly shaped charged bodies. Nor is it necessary that the curved static bar completely surround the charged body. For example, a semi-circular orparabolically shaped static bar is also useful for some applications. 7

In FIG. 2 there is shown a perspective view of one embodiment of the invention. There is shown generally a static bar 10 comprising a frame 12 having a curvature in its long axis and which is annular in shape; and

,- in this view the static eliminator device forms a complete ring. However, as stated above, it is not always necessary that the static eliminator be in the form of a ring; for example, the static eliminator device may be formed to any shape which would most effeciently remove the static from an irregularly shaped body. The static bar usually is provided with suitable mounting brackets 11.

In FIG. 3 there is shown a cross-sectional view taken along line 3-3 of FIG. 2 showing a frame 12 which is slightly U-shaped in cross-section, the open end of the U being directed toward the center of the annulus. Positioned within frame 12 is a layer 14 of radioactive source material for emitting ionizing radiation to eliminate static charges from a charged body. When the cross-section of frame 12 is thus U-shaped, the radiation from source 14 is emitted in a direction toward the charged body and is prevented from radiating in all directions. Thus, the radioactive static eliminator is safe in use because the radiation is directed only at the charged body. The energy-emitting face 16 of the U- shaped frame is provided with a metal screen member 18 to prevent damage to the radioactive source while allowing radiation to pass through freely.

The radioactive source material 14 conveniently is comprised basically of a compact monolayer of a multitude of discrete microspheroidalparticles (not shown) containing, e.g., about 20-25 millicuries of polonium errsotsr length l fjthei 'raaiaaataa"iii siesta can also be used, e.g., Americium Krypton* (contained in a tube), promethium and the like.

lnFlG. 4 there is shown another embodiment of a curved static eliminator having a curvature in the long axis of the frameand radioactive source. In this embodiment the device. is helical static eliminator 20 for use around elongated charged bodies. For example, this type of static eliminator device would be very desirable for use around a plastic tube through which powdered material is being passed and also for use around a plastic bag which is being filled with powdered material. Reduction of the static charge around the tube or bag would result in more efficient control of the flow of the powder and in more effective heat sealing of such bags.

The curved static eliminators of the invention may be made by a number of methods. Particularly desirable radioactive spheroids useful for radioactive source material in the practice of the invention are those formed by dispersing a radioactive isotope by ion exchange within pores of an inorganic matrix having a bead shape, and then fixing the radioactive material within the pores by a beating step effective to cause shrinkage of the pores and trapping of the radioactive material, as described in U.S. Pat. No. 3,147,225 (issued to J. P. Ryan, Sept. 1, 1964 The so-formcd radioactive spheroids may then be mounted on or fastened to a deformable substrate. After the spheroids have been so mounted, the substrate may be formed to the desired shape. For example, the spheroid particles may first be mounted on an aluminum strip with an epoxy resin, and then the aluminum strip may be formed to a curved shape and placed within a curved frame.

The radioactive spheroid particles may be mounted on a deformable substrate, or directly to a curved frame, by any of several methods. For example, the spheroid particles may be forcibly pressed into a duetile metal strip, e.g., aluminum strip, as described in U. S. Pat. No. 3,376,422 (issued to D. L. Haes, Apr. 2, 1968 or they may be adhesively bonded to metal or plastic, e.g., polyimide or polyethylene terephthalate. Another useful technique is to place the spheroids on the desired substrate and then paint over them using a silk screen and ceramic'paint.

Preferably the radioactive microspheroidal particles are comprised of polonium-2 l 0, as these particles emit alpha radiation which is less dangerous than beta or gamma radiation. It has been found that the alpha particles emitted from the polonium-2l0 source travel only about 4 centimeters in ambient air. Thus, preferably the curved static bars of. the invention are positioned not farther than about 4 cm. from the surface of the charged body for most effective static removal. Accordingly, the, preferred annular static bars have a diameter less than about 8 inches, although static bars with diameters of ID to .12 inches are also useful, depending on the size and shape of the charged body.

The effectiveness of the charged static eliminators of the invention is illustrated by reference to the following examples.

Example I A charged polyethylene web having a semi-circular cross-section with a radius of curvature of about one inch was passing intoa web processing machine at the rate of 400 feet per minute. Web potential measurements indicated that the static charge on the web was 15,000 volts.

When a curved static eliminator having a semicircular curvature in its long axis was positioned around the curved web so as to conform generally to the curved surface thereof and at a distance of about 1 inch therefrom at all points, the static charge was reduced to essentially zero volts. When a conventional flat or noncurved radioactive static eliminator was positioned 1 inch from a similarly charged (i.e., 15,000 volts) curved polyethylene web traveling at 400 feet per minute, the static charge was only reduced to 2500-3000 volts.

I Example II A continuous PVC tubing of 1% inchdiametentraw cling at 300 feet per minute, carried a static charge of 30,000 volts. When a conventional flat radioactive static eliminator was positioned at a distance of 1% inch from the tubing, the static was reduced to 5,000 volts. However, when an annular static eliminator with a four inch diameter was positioned coaxially with respect to the charged tubing, the static charge was reduced from 30,000 volts to zero volts.

Example lll Another application wherein the curved static eliminator has exhibited unexpected utility is where an annular static eliminator is fitted onto the barrel ofa combination fiber-chopping and spray gun used for fiberglass. The gun has a rubber roller which feeds strands of fiberglass into the gun where theyare cut into short fibers. Because of the frictional engagement between the rubber roller and the fiberglass strands, a strong static charge is generated which causes the glass fibers to stick to one another and-to any grounded body. A further annoyance is that it is difficult to obtain uni form spraying and coating quality because of the static charge on the fibers. When a ring or annular shaped static eliminator was fitted to the barrel of the combination fiber-chopping and spraying gun, the static charge on the out-coming fibers was essentially completely eliminated. As a result, of course, uniform spraying of the fibers was possible and the annoyance of clinging fibers was eliminated.

Example IV 3,000-l0,000 volts, on the polyethylene tube material,

tors have proven useful include static elimination frompipe coating operations, grinding operations, ionization of large volumes of air being passed through a ringshaped static eliminator, a'ndthe like.

What we claim is:

l. A device for the removal of electrostatic charge from a charged body, said device comprising, (a) a frame, and (b) a radioactive source positioned within said frame; said frame and saidradioactive source having a curvature in the long axis thereof and forming a structure adapted to coaxially encircle at least part of said charged body, said structure having a curvature which is at least semi-circular, and said device being adapted to emit radiation in a direction toward said charged body.

2. The device of claim 1 wherein the radioactive source comprises polonium-2l0 in the form of a compact monolayer of a multitude of discrete microspheroidal particles. 7

3. The device of claim 2 wherein the frame is U- shaped in cross-section.

4. The device of claim 3 wherein the frame is annular in shape.

5. The device of claim 3 wherein the frame is helical in shape. I

6. The device-of claim 4 wherein the diameter of the fgame is less than about 8 inches. "W p 7. The device of claim 1 wherein said frame is adapted to partially surround and generally conform to the cross-sectional configuration of said charged body. =l k 

1. A device for the removal of electrostatic charge from a charged body, said device comprising, (a) a frame, and (b) a radioactive source positioned within said frame; said frame and said radioactive source having a curvature in the long axis thereof and forming a structure adapted to coaxially encircle at least part of said charged body, said structure having a curvature which is at least semi-circular, and said device being adapted to emit radiation in a direction toward said charged body.
 2. The device of claim 1 wherein the radioactive source comprises polonium-210 in the form of a compact monolayer of a multitude of discrete microspheroidal particles.
 3. The device of claim 2 wherein the frame is U-shaped in cross-section.
 4. The device of claim 3 wherein the frame is annular in shape.
 5. The device of claim 3 wherein the frame is helical in shape.
 6. The device of claim 4 wherein the diameter of the frame is less than about 8 inches.
 7. The device of claim 1 wherein said frame is adapted to partially surround and generally conform to the cross-sectional configuration of said charged body. 